1. Field of the Invention
The present invention relates generally to a developing device for developing an electrostatic latent image formed on an electrostatic latent image support member using toner particles in an image forming apparatus such as a printer, and more particularly, to a developing device using a monocomponent developer (referred to as monocomponent developing device hereinafter) so adapted that a toner conveying member for conveying toner particles to an electrostatic latent image support member is brought into contact with the surface of the electrostatic latent image support member and a developing bias voltage is applied to the toner conveying member from a power supply to supply toner particles charged to the same polarity as that of an electrostatic latent image formed on the electrostatic latent image support means to the electrostatic latent image support member from the toner conveying member to develop the electrostatic latent image by reversal development.
2. Description of the Prior Art
As a developing device used in an image forming apparatus such as a printer, a monocomponent developing device using only toner particles as a developer has been conventionally known in addition to a developing device using a two-components developer, that is, using toner particles and a carrier as a developer (referred to as two-components developing device hereinafter).
In recent year, the above described monocomponent developing device has received attention because its structure is generally simpler so that it can be made smaller in size and lower in cost, as compared with the two-components developing device.
Examples of such a monocomponent developing device generally include one so adapted that toner particles are supplied to the surface of a toner conveying member, a blade is brought into contact with the surface of the toner conveying member thus supplied with the toner particles by applying pressure to charge the toner particles supplied to the surface of the toner conveying member by triboelectric charging as well as to control the amounts of the toner particles supplied to the surface of the toner conveying member and then, the toner particles thus charged by triboelectric charging are conveyed to an electrostatic latent image support member by the toner conveying member, and the toner conveying member is brought into contact with the surface of the electrostatic latent image support member to supply the toner particles to the electrostatic latent image support member from the toner conveying member.
In the monocomponent developing device, however, when the toner conveying member is brought into contact with the electrostatic latent image support member, such a phenomenon occurs that the toner particles are supplied to the electrostatic latent image support member from the toner conveying member even when the potential difference between the toner conveying member and the electrostatic latent image support member is zero volt, unlike the two-components developing device using toner particles and a carrier as a developer.
The foregoing will be described in more detail. In the two-components developing device using toner particles and a carrier as a developer, when toner particles 1 are negatively charged, a carrier 101 is positively charged, as shown in FIG. 1. That is, the toner particles 1 and the carrier 101 are respectively charged to opposite polarities so that the entire developer has no charges.
In the above described two-components developing device, therefore, when both the surface potential of an electrostatic latent image support member 2 and the potential of a toner conveying member 15 are zero volt, the potential of the developer between the electrostatic latent image support member 2 and the toner conveying member 15 becomes a straight line connecting the surface potential of the electrostatic latent image support member 2 and the potential of the toner conveying member 15, that is, has no gradient, as shown in FIG. 1. Consequently, the toner particles 1 are not moved to the electrostatic latent image support member 2 from the toner conveying member 15.
On the other hand, in the above described monocomponent developing device, the developer is constituted by only toner particles 1 charged either negatively or positively, as shown in FIG. 2. Accordingly, even when both the surface potential of an electrostatic latent image support member 2 and the potential of a toner conveying member 15 are zero volt, the potential of the developer between the electrostatic latent image support member 2 and the toner conveying member 15 is expanded in the direction of polarity to which the toner particles 1 are charged between the electrostatic latent image support member 2 and the toner conveying member 15, as shown in FIG. 2. Consequently, part of the toner particles 1 are supplied to the electrostatic latent image support member 2.
When the monocomponent developing device is driven from a state where it is stopped with the toner conveying member 15 holding the toner particles 1 being in contact with the surface of the electrostatic latent image support member 2, as shown in FIG. 3, the toner conveying member 15 and the electrostatic latent image support member 2 are rotated and a charger 3 is turned on, to charge the electrostatic latent image support member 2 by the charger 3.
When the developing device is thus driven, a portion of the above electrostatic latent image support member 2 on the downstream side of the position opposed to the above charger 3 in the direction of rotation of the above electrostatic latent image support member 2 is not charged by the charger 3, so that the surface potential of the portion is zero volt. Further, in a state where a power supply 18 connected to the toner conveying member 15 is turned off, the potential of the toner conveying member 15 is also zero volt.
Therefore, when a portion .theta. of the above described electrostatic latent image support member 2 on the downstream side of the position opposed to the charger 3 in the direction of rotation of the electrostatic latent image support member 2 to a point in contact with the toner conveying member 15 is brought into contact with the toner conveying member 15 holding the toner particles 1 by the rotation of the electrostatic latent image support member 2 and the toner conveying member 15, the toner particles 1 are supplied to the electrostatic latent image support member 2 from the toner conveying member 15 in the above described portion .theta. of the electrostatic latent image support member 2, as described with reference to FIG. 2, because both the surface potential of the electrostatic latent image support member 2 and the potential of the toner conveying member 15 are zero volt.
In the above described monocomponent developing device, consider a case where the above toner particles 1 are charged to opposite polarity to that of an electrostatic latent image formed on the electrostatic latent image support member 2 to develop the electrostatic latent image by normal development. In this case, if the above power supply 18 is turned on to apply to the toner conveying member 15 a developing bias voltage having opposite polarity to that of the toner particles 1 simultaneously with the driving of the developing device, the toner particles 1 can be prevented from being supplied to the electrostatic latent image support member 2.
On the other hand, in the above described monocomponent developing device, consider a case where the toner particles 1 are charged to the same polarity as that of the electrostatic latent image formed on the electrostatic latent image support member 2 to develop the electrostatic latent image by reversal development. In this case, if the power supply 18 is turned on in the same manner as the case of the above described normal development, a developing bias voltage having the same polarity as that of the toner particles 1 is applied to the toner conveying member 15, so that the toner particles 1 are further supplied to the electrostatic latent image support member 2.
In the case of the above described reversal development, therefore, the power supply 18 must not be turned on, because as a result of this, the toner particles 1 are supplied to the portion .theta. whose surface potential is zero volt of the electrostatic latent image support member 2 as described above to be consumed wastefully.
In order to solve the above described problem, inventors of the present application came up with the idea of using as the power supply 18 one capable of selecting two types of voltages in the developing device shown in FIG. 3 to apply a voltage having opposite polarity to that in the case of reversal development to the toner conveying member 15 from the above power supply 18 until the portion .theta., which is not charged by the charger 3, of the electrostatic latent image support member 2 passes through the point in contact with the toner conveying member 15 when the developing device is driven.
However, the above described power supply 18 capable of selecting two types of voltages is high in cost, and an operation for switching voltages in the above power supply 18 becomes complicated.
Then, the inventors of the present application came up with the idea of providing a switching device 4 between the toner conveying member 15 and the power supply 18 to turn the switching device 4 off using control means such as a relay so that the toner conveying member 15 is placed in an electrically floating state until the portion .theta., which is not charged by the charger 3, of the electrostatic latent image support member 2 passes through the point in contact with the toner conveying member 15 as described above at the time of starting the developing device.
In a case where the switching device 4 is thus turned off so that the toner conveying member 15 is placed in an electrically floating state until the portion .theta., which is not charged by the charger 3, of the electrostatic latent image support member 2 passes through the point in contact with the toner conveying member 15, when the toner particles 1 negatively charged are supplied to the electrostatic latent image support member 2 from the toner conveying member 15, positive charges having opposite polarity to that of the toner particles 1 are produced on the surface of the toner conveying member 15, as shown in FIG. 5. The positive charges are maintained in the toner conveying member 15 without flowing toward the power supply 18 from the toner conveying member 15.
Consequently, the potential of the toner conveying member 15 is instantaneously raised to a voltage V.sub.1 corresponding to the positive charges thus maintained in the toner conveying member 15 from zero volt, as shown in FIG. 6, due to the positive charges. Accordingly, the toner particles 1 are attracted to the toner conveying member 15, not to be supplied to the electrostatic latent image support member 2.
When the switching device 4 is turned on or off by the control means such as a relay as described above, however, the control becomes complicated and the stable control is difficult so that the control lacks reliability.
The inventors of the present application further study the above described monocomponent developing device for developing the electrostatic latent image by reversal development so as to prevent the toner particles from being supplied to the electrostatic latent image support member from the toner conveying member to be wastefully consumed at the beginning of driving the developing device, leading to the completion of the present invention.